This invention relates generally to television signal transmission systems and methods and specifically concerns a novel spectrum compatible High Definition Television (HDTV) signal encoding and transmission system.
Recently, extensive interest has been generated in developing a so-called High Definition Television system having higher picture definition than existing television systems and improved audio. Such a system should also desirably exhibit improved noise performance and may have a wider aspect ratio. A difficulty is in the available spectrum space for terrestial broadcasting of television signals. It is desirable that HDTV television receivers be capable of receiving appropriate video and sound signals while the existing population of NTSC receivers continue to receive their accustomed NTSC performance without perceptible impairment.
The proposed HDTV systems may generally be categorized into three groups; compatible systems, augmentation systems and non-compatible systems. Compatible systems add to or modify an existing NTSC transmission in a manner so that it may still be viewed on an existing receiver with NTSC resolution, but may be received with higher resolution on a special HDTV receiver. The compatible transmission is normally limited to the same 6 MHz channel used for NTSC transmissions. Augmentation systems, on the other hand, transmit augmenting signals in additional spectrum space without disturbing the NTSC transmission. Thus, an existing receiver may reproduce the televised image with NTSC resolution by tuning the standard channel, while a special HDTV receiver may be used to tune both the standard channel and the augmenting signals to reproduce an HDTV image. Transmissions comprising various combinations of these two techniques are also possible.
It is believed that a non-compatible system will provide the best solution to the problem of providing high quality HDTV. Compatible systems have the disadvantage of almost inevitably producing undesired artifacts in standard NTSC receivers. Augmentation systems have the disadvantage that multiple RF bands must be tuned and that different signals must be precisely pieced together to form the HDTV image. Non-compatible systems provide HDTV transmissions which cannot be received by standard NTSC receivers. Since such transmissions must be made over unused spectrum, care must be exercised to avoid interference with existing NTSC channels. Co-channel and adjacent channel interference are of particular concern in this regard.
With respect to spectrum availability, it is well known that many of the designated television signal channels are not used in a particular geographical area. This is due to the impossibility of adding new transmitted signals in the unassigned positions of the spectrum because of a large number of FCC mandated prohibitions (particularly in the UHF band), colloquially referred to as "taboos." In most metropolitan areas, spectrum utilization is restricted to every second VHF channel and every sixth UHF channel. It will be seen that with the transmission system of the invention, the transmission format is changed (with corresponding changes in receiver requirements) to enable more efficient utilization of the existing VHF/UHF spectrum. With the invention, every existing NTSC broadcast station will be capable of obtaining a second 6 MHz channel over which an HDTV signal can be broadcast simultaneously with the existing NTSC program. Thus with the system of the invention, a spectrum compatible system with existing NTSC signals is obtained.
The preferred embodiment of the invention provides special benefits with regard to adjacent channel and co-channel interference problems with NTSC and other type television signal transmission and receiving systems. The improved transmission system permits operation in or adjacent to NTSC television signal transmission areas without objectionable adjacent channel and co-channel interference, either into or from the NTSC channel.
An NTSC television signal occupies a 6 MHz bandwidth and imposes significant transmission power demands. These power demands are directly related to the cost of operating the signal transmitter and reductions therein can yield significant economic benefit. Also of great importance is the fact that cable television plants, especially those of older vintage, are restricted in the number of channels they can handle by the signal power handling capabilities of their amplifiers. It would be highly desirable to reduce the amount of signal power required to transmit television signals, thereby reducing transmitter operating costs and permitting a larger number of television channel signals to be handled by a cable plant of given power handling capability. The system of the invention achieves a marked reduction in the power required to transmit an AM television signal without discernible degradation of signal fidelity and therefore provides a solution to these needs of the prior art.
In its FM implementation, the system of the invention enables transmission with significantly less bandwidth since the deviation of the FM signal is minimized, which will find ready application in Direct Broadcast Satellite (DBS) transmission systems. The smaller bandwidth directly improves the signal to noise performance of the system, which improvement may be translated into smaller receiving antennas. Thus, the system of the invention will be seen to solve a longstanding problem in DBS transmission systems.
The inventive transmission system has a number of important aspects. In accordance with a fundamental aspect thereof, a television signal is configured such that the transmitted signal is a "hybrid," that is; it has a coded (digital) portion representing signal components of low picture detail and relatively high transmission power demand that may be transmitted in a relatively low power utilization format; and an analog portion, representing signal components of high picture detail and relatively low transmission power demand. The demarcation between the analog and digital portions is a function in part of the availability of means to transmit the digital data. In accordance with the invention, the data is sent in non-active video portions of the transmitted signal.
The hybrid system is subdivided into a "basic hybrid," in which the removed and digitized low frequencies are under about 15 KHz and an "extended hybrid" in which the removed and digitized signal frequencies are under about 200 KHz. As will be explained in detail below, in the basic hybrid form of the invention, the digital part comprises video components below the line deflection frequency that are digitally coded and transmitted as data during non-active video portions of the television signal. In the extended hybrid form of the invention, the digital part comprises video components below about 200 KHz that are digitally coded and transmitted as data during non-active video portions of the television signal. Since the invention may be used with many different television signal formats, the non-active video portions of the signal may include either or both the horizontal and vertical blanking intervals.
It has also been found that further benefits are obtained by sequentially applying basic hybrid processing and extended hybrid processing (referred to as two step processing), with the basic hybrid processing being performed for the active video of each horizontal line such that the low frequency average of each horizontal line is removed from the analog signal. The remaining components below 200 KHz are subsequently removed.
A further aspect of the inventive system involves "temporal pre-emphasis," also referred to as temporal filtering, field processing or frame combing. With this approach, transmission power for stationary images is reduced while transmission power for moving images is increased. Since the average television picture is, relatively speaking, static, the use of temporal pre-emphasis is of benefit because the greater interference potential of signals corresponding to moving images is outweighed by the fact that noise in a moving image is much less noticeable than noise in a stationary image. Temporal de-emphasis is applied in the receiver. This aspect of the invention has advantages in any "video" transmission system independent of the hybrid processing of the signal. This is due to the fact that normally there is little change between successive frames of video and emphasizing the changes relative to the static portions results in very efficient transmission.
Yet another aspect of the inventive system involves compressing the "hybrid" video signal to achieve a large signal to noise ratio for broad, flat video areas, where noise is readily discernible, and a low signal to noise ratio for narrow video components, representing edges and video detail, in which noise is much less discernible. In the receiver, the signal is expanded to undo the compression in the transmitter. The combination of compression and expansion is referred to as "companding."
Still another important feature of the inventive system is the use of dispersal filtering to reduce the amplitudes of the peak video components by distributing their energy among the voids created in the hybrid video signal. As will be seen, these voids are the direct result of hybrid processing of the video signal whereby low frequency analog components are removed, coded and included as data in the non-active video portions of the remaining analog high frequency components.
As those skilled in the art will readily perceive, reduction of the average power of the transmitted signal is highly desirable, especially where adjacent channel and co-channel interferences are concerned. The reduction occurs because of the hybrid processing of the video signal which effectively replaces low frequency video signals with "doublets" that define the edges of the video image. With temporal filtering, the largest signals result from moving video edges which can be compressed even more. Companding increases the signal to noise performance for relatively stationary edges of video images at the expense of much less observable noise associated with moving video image edges. Dispersal filtering primarily reduces the amplitudes of the signals above the hybrid processing frequency range.
Other advantages flow from application of the various aspects of the inventive transmission system that improve signal to noise performance, especially with respect to the ability to operate in an environment of adjacent and co-channel NTSC signals. These include; the technique of precise carrier frequency offset with respect to co-channel NTSC signals to cause "break up" of interfering signals and thereby reduce their visibility in the video display; and co-location (locating the hybrid signal transmitter of the invention close to the adjacent channel NTSC transmitter) to assure that receivers in both reception areas receive approximately equal strength signals to enable their respective AGC systems to set up properly. Also frame locking the hybrid signal to the NTSC signal and incorporating all data in the vertical blanking interval of the hybrid signal contributes to the ability to operate in a co-channel environment. It will, of course, be clear that the many aspects of the inventive system may have benefits that are independent of other aspects of the system, and that the use of one or more of the aspects in combination with each other produces even greater benefits.
The encoding format used in the HDTV system of the present invention has substantially improved resolution relative to the current NTSC system. While being non-compatible with NTSC, it affords a viable solution to the search for a high quality HDTV system that will not obsolete the existing population of NTSC receivers. The encoding format further minimizes adjacent channel and co-channel interference problems by exhibiting dominant timing periodicities equal to those characteristic of an indigenous television system, such as NTSC in the United States.
While the techniques of the invention may be used with non-compatible, as well as augmentation-type HDTV systems to allow maximum utilization of existing spectrum, they are of particular benefit when used with Zenith Electronics Corporation's Spectrum Compatible High Definition Television System presently under consideration.